In vehicles, a catalyst body is provided midway along an exhaust passage in an exhaust system in order to purify exhaust emissions which are discharged from an internal combustion engine. In some of the internal combustion engines, there is provided a catalyst deterioration-detecting device which includes a control means for determining the deterioration of the catalyst body when predetermined catalyst deterioration-determining conditions are established. The control means provides first feedback control of an air-fuel ratio toward a target value in accordance with a first voltage signal which is output by a front oxygen sensor. Further, the control means executes second feedback control of the air-fuel ratio so as to correct the first feedback control in accordance with a second voltage signal which is output by a rear oxygen sensor. The front oxygen sensor, which is a first exhaust sensor, is disposed in an exhaust passage of the engine on an upstream side of the catalyst body. The rear oxygen sensor, which is the second exhaust sensor, is disposed in the exhaust passage on a downstream side of the catalyst body.
Examples of the above detecting device are disclosed, e.g., in published Japanese Patent Applications Laid-Open Nos. 5-240089 and 6-81634.
According to Laid-Open No. 5-240089 (and corresponding U.S. Pat. No. 5,337,557), second feedback control of the rear oxygen sensor is subjected to change in a correction-determining time and a correction quantity in accordance with a state of output periods of a second detection signal from the rear oxygen sensor. A second feedback control-learning value of the rear oxygen sensor is calculated from: an arithmetic mean, which is calculated from both a previous skip value ante-value and a present skip value ante-value for each skipping of a second feedback control value; and, an arithmetic mean value which is calculated in accordance with a state of the output periods of the aforesaid second detection signal. As result, the above calculated learning value provides feedback control of an air-fuel ratio.
According to the aforesaid Japanese Patent Application Laid-Open No. 6-81634 (and corresponding U.S. Pat. No. 5,379,587), when predetermined deterioration judgement-executing conditions are satisfied, both period and area ratios of first and second detection signals within a predetermined arithmetic operating time are corrected by a correction value, thereby producing a deterioration-judging arithmetic value. The arithmetic value is used to make a calculation so as to determine a degraded State of the catalyst body. In this way, the area ratio as well as the period ratio of the first and second detection signals are calculated and multiplied together; and, the deterioration-judging arithmetic value, which is corrected by a correction value, is obtained to pass a judgment. As a result, it is possible to precisely measure a degraded state of the catalyst body, thereby providing improved accuracy in determining the degraded state. In short, according to Application No. 6-81634, the deterioration-judging arithmetic value (REKCAT), which is catalyst deterioration-judging/measured values, is determined from: REKCAT=SR.times.SHUKI.times..alpha., where SR is an area ratio, SHUKI is a period ratio, and .alpha. is a correction factor according to an exhaust temperature, and engine load, and the like.
In the catalyst deterioration-detecting device for the internal combustion engine, the catalyst body does not materially detract from its function so far as vehicles which are in normal use.
However, when a vehicle user uses, e.g., leaded fuel, or when a misfire results from a high-tension cord being pulled out of position as a result of other unexpected causes, the function of the catalyst body is dramatically reduced by either diminished catalytic function or damaged catalyst body due to lead-causing poison or high temperature. The reduced function of the catalyst body reduces the exhaust-purifying rate.
This causes an inconvenience in that a large amount of unpurified exhaust gas is liberated in the air, which contributes to environmental aggravation.
Further, low precision in determining catalyst deterioration causes the catalyst body to be judged as abnormal in spite of being normal. This causes inconveniences in that users experience a feeling of uneasiness, with a consequential loss of the reliability of vehicles, and further the users are urged to do needless repair of vehicles or unnecessary replacement of parts, with a concomitant increase in both an after-sales-service man hour and repairing cost.
In order to obviate the aforesaid inconveniences, the present invention provides a catalyst deterioration-detecting device for an internal combustion engine, having first and second exhaust sensors disposed in an exhaust passage of the internal combustion engine respectively on upstream and downstream sides of a catalyst body, the catalyst body being placed generally midway along the exhaust passage, which catalyst deterioration-detecting device effects first feedback control of an air-fuel ratio toward a target value in accordance with a first voltage signal which is output by the first exhaust sensor, while performing a second feedback control of the air-fuel ratio so as to correct the first feedback control in accordance with a second voltage signal which is output by the second exhaust sensor, whereby the deterioration of the catalyst body is determined when predetermined catalyst deterioration-determining conditions are fulfilled, the improvement comprising: a control means having a catalyst deterioration-determining section which determines a degraded state of the catalyst body, when the predetermined catalyst deterioration-determining conditions are established, by taking the sequential steps of: respectively measuring first and second voltage signal-inverted states within a predetermined arithmetic operating time in accordance with inverted states of first and second voltage signals, thereby calculating an inverted state ratio; respectively measuring first and second voltage signal-surrounding areas within the predetermined arithmetic operating time in accordance with respective areas which are surrounded by loci of periods of time during which the first and second voltage signals are inverted, thereby calculating an area ratio; respectively measuring first and second voltage signal states within the predetermined arithmetic operating time in accordance with the first and second voltage signals, thereby calculating a voltage ratio; calculating a catalyst deterioration-Measured value on the basis of the inverted state ratio, the area ratio, and the voltage ratio; and, comparing the catalyst deterioration-measured value and a catalyst deterioration-determining value, the latter value being set for each engine load.
According to the structure incorporating the present invention, when predetermined catalyst deterioration-determining conditions are met, the catalyst deterioration-determining section of the control means take the successive steps of: respectively measuring the first and second voltage signal-inverted states within a predetermined arithmetic operating time in accordance with inverted states of the first and second voltage signals, thereby calculating an inverted state ratio; respectively measuring the first and second voltage signal-surrounding areas within the predetermined arithmetic operating time in accordance with respective areas which are surrounded by loci of periods of time during which the first and second voltage signals are inverted, thereby calculating an area ratio; respectively measuring the first and second voltage signal states within the predetermined arithmetic operating time in accordance with the first and second voltage signals, thereby calculating a voltage ratio; calculating a catalyst deterioration-measured value on the basis of the inverted state ratio, the area ratio, and the voltage ratio; and, comparing the catalyst deterioration-measured value and the catalyst deterioration-determining value, the latter being set for each engine load. The determining section thereby determines a degraded state of the catalyst body. The use of the preceding control means provides simulative determination as to a degraded state of the catalyst body, thereby enabling accurate judgment on a state of catalyst deterioration, even in consideration of dispersion in measurements as well as vehicles/parts. Furthermore, the degraded state of the catalyst body is determined with reference to the catalyst deterioration-measured value, thereby making it possible to improve the accuracy of judgment on catalyst degradation. Moreover, the reliability of vehicles is enhanced, which further avoids needless repair or replacement of parts, Yet further, environmental aggravation can be prevented because precise repair is achievable when the catalyst body is abnormal.